<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
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    <title>Document</title>
    <script src="https://cdn.bootcss.com/jquery/3.4.1/jquery.min.js"></script>
    <script>
        $(function() {
            $("form input").css("border", "deeppink 1px solid");
        });
    </script>
    <style>
        .input {
            border: deeppink 1px solid
        }
    </style>
</head>

<body>
    <form>
        <fieldset>
            <label>包含子文本框
                <input></input>
            </label>
            <fieldset>
                <label>包含孙文本框
                    <input></input>
                </label>
            </fieldset>
        </fieldset>
    </form>
</body>

</html>